In a drug delivery device, a piston within a cartridge that contains a drug may be displaced with respect to the cartridge in the distal direction by a piston rod which moves in the distal direction with respect to the cartridge.
A fixed dose pen injector is a device that can be used to inject a number of set dose sizes from a pre-filled cartridge which may be made of glass. This may be ideally suited for chronic daily therapies where repeat doses of the same size are regularly required. The device may be disposable or reusable.